1. Field of the Invention
The present invention relates to the fields of optical and/or fluorescence microscopy, scanning probe microscopy, photolithography, and semiconductor manufacturing and quality control. Specifically, the present invention provides methods for detecting and imaging subwavelength size structures.
2. Description of the Related Art
The interaction of light with nano-apertures in metallic films produces interesting effects that resemble the properties of traditional optical elements, such as chromatic filters (1-11) and lenses (12-14). The optical characteristics of these perforated films are not completely understood and appear to be created by the interference of the transmitted light with surface modes, plasmon polaritrons and/or diffracted evanescent waves (1-11, 15). Experimental methods used for imaging the transmittivity of subwavelength apertures utilize near-field scanning optical microscopy (NSOM). NSOM has been used to characterize the optical properties of a variety of apertures: nanoholes (16), arrays of nanoholes (17), nanoslits (12,18), and apertures surrounded by periodic corrugations (12). However, unlike the surface near-fields, the 3D far-field light distribution is far more difficult to determine experimentally and hence there is little data available to confirm theoretical predictions.
One experimental profiling technique has been used to map the far-field light distribution emanating from an illuminated nanoslit (19). This technique currently has a spatial resolution of 5 μm and is restricted to 2D beam profiling. Another way to understand light interactions with nano-apertures is through numerical simulations of Maxwell's equations, such as the finite-difference time-domain (FDTD) method (13,14,16-18,20). The FDTD calculated near-fields surrounding the apertures are in impressive agreement with experimental observations (16-18). But, direct numerical calculations of the 3D far-field light distributions are seldom performed because of the enormous amount of computational resources required.
Optical microscopy, both transmission and fluorescence, has limited spatial resolution due to well-known limits of diffraction. Few methods are available to increase resolution. For example, electron microscopy increases resolution, but requires samples be treated and that samples are non-living. Furthermore, approaches to increase optical resolution are complex and expensive, for example, the use of stimulated emission depletion (STED) microscopy, which uses multiple lasers.
Pendry described a lens based on negative refraction materials (21). However, such materials do not exist in the optical region. Furthermore, even if such materials are created the images exist only in the near-field above the metal, approximately 50 nm (22). The imaging methods presented herein also are distinct from the Talbot-effect because the known effect does not result in subwavelength features (23).
Thus, there is a recognized need in the art for methods of high resolution optical mapping of structures with sub-optical wavelength dimension that do not require contact or near-field measurement. More specifically, the prior art is deficient in methods of imaging subwavelength structures via mapping the three dimensional spatial distribution of light transmitted through a structure of interest at far-field distances from the surface. The present invention fulfills this long-standing need and desire in the art.